The fuel assemblies used at the present time in such reactors generally comprise a bundle of fuel rods (this term designating not only rods loaded with fissile material but also those loaded partially or completely with fertile material) and a structure holding the bundle, comprising upper and lower end pieces connected together by guide tubes which carry grids for maintaining the fuel rods at the nodal points of a regular network.
The grids are distributed along the assembly, at sufficiently close intervals to maintain a regular spacing between the fuel rods over their whole length, despite deformations caused by irradiation, and to give a high value to the natural vibration frequencies of the rods between two grids, which avoids energization of dangerous vibratory working conditions by the coolant.
Considered as a whole, the grids must fulfil two other functions:
supporting rods, PA1 mixing the flow streams about the fuel rods, so as to avoid the appearance of hot spots.
The first function is generally fulfilled by one and/or the other of the endmost grids, provided with springs for clamping the fuel rods. The other function is fulfilled by at least intermediate grids.
Different types of grids are already known for maintaining the fuel rods at the nodal points of the triangular network.
There is known in particular (FR-A-2594998) a grid comprising a hexagonal shaped belt and three sets of plates fixed to the belt, the plates of each set being mutually parallel and forming an angle of 120.degree. with the plates of the other sets.
In that prior art grid, the plates of one set form a bed spaced apart from the other beds in the longitudinal direction of the assembly. This approach has a number of advantages; on the other hand, since the plates are all parallel to a face of the belt and extend throughout the width of the grid, it is necessary to provide several plate patterns, which considerably complicates the manufacture.
A fuel assembly is also known (FR-A-2145676 or EP-A-0239441) having an external casing extending throughout the assembly, in which structures for holding the rod in position are spaced apart longitudinally at even intervals. Each structure comprises two sets of plates, the plates of one set being parallel to two mutually opposite faces of the case and forming an angle of 120.degree. with those of the other set, so as to define cells having a diamond-shaped section whose axes are distributed in a triangular network. The plates of one set are offset longitudinally with respect to those of the other set and the two sets are fast with the casing. This approach involves the presence of a casing. The structures are not isotropic mechanically. Again the structure comprises a large number of different plates.